Threading or (pseudo-)forking? (It kinda looks like you have two threads running the tests simultaneously.) Or just confused test numbers?

Either way, you are getting the same test numbers twice in the test stream:

...
ok n
ok n
ok n+1
ok n+1
...
[download]

I ran into this a lot when writing tests that fork. Mostly I resolved it by manually increasing the test count and waiting for the children to terminate:

# in the parent, while a child is running $n tests:
my $Test = Test::More->builder;
$Test->current_test( $Test->current_test + $n );
wait;
[download]

Could we perchance see the .t code that produces these complaints?

Update: As andreas1234567's code demonstrates, the error message does not mean what I thought it meant (and still think it says). An "ok 2" followed by an "ok 1" will produce "test 2 answered after test 2". (And an "ok 1" followed by another "ok 1" are quietly accepted as two different tests. Bah.)

Oh well, it's still a case of test numbers coming in the wrong order. (And meanwhile, a new harness is on its way ...)

$ cat 641197.t
use strict;
use warnings;
use Test::More qw(no_plan);
my $pid = fork;
($pid) ? print "ok 1\n" : print "ok 2\n";
__END__

$ prove 641197.t
641197....ok 1/0Confused test output: test 2 answered after test 2
# No tests run!
641197....dubious
        Test returned status 255 (wstat 65280, 0xff00)
FAILED--1 test script could be run, alas--no output ever seen
$
[download]

TFT Andreas,

However, there are no explicit calls to fork() in my test script ... nor the code under test ?!

The really odd thing is that until yesterday, the module wasn't working and I wasn't seeing the errors - however, since I managed to get the module to work, the errors manifested themselves ... the principle change being the adoption of the Hash::Merge module in favour of a homegrown attempt - AFAICT, the Hash::Merge module only employs recursion i.e. I don't believe it to be using fork().

Yes, a recursive function can also produce similar results:

$ cat 641197-2.t
use strict;
use warnings;
use Test::More qw(no_plan);

sub f {
    my $i = shift;
    print "ok $i\n";
    return $i < 2 ? 1 : f($i-1) + f($i-2);
}

f(3);
__END__

$ prove 641197-2.t
641197-2....# No tests run!
641197-2....ok 1/0Confused test output: test 2 answered after test 3
Test output counter mismatch [test 4]
Test output counter mismatch [test 5]
641197-2....dubious
        Test returned status 255 (wstat 65280, 0xff00)
FAILED--1 test script could be run, alas--no output ever seen
[download]

--
Andreas